Vapor generators

ABSTRACT

Apparatus for the generation of liquid vapor under pressure by means of heat from the combustion products of an internal combustion engine comprises a reservoir for the liquid to be vaporized having an inlet duct for the combustion products which communicates with the lower part of the interior of the reservoir. The combustion products pass from the inlet duct into the reservoir by way of a screen formed with a large number of small holes providing free passage for gas or liquid and this screen is preferably formed by an extension of the duct lying within the reservoir and formed with the small holes round its periphery. As a result of this construction the combustion product may bubble upwardly through liquid in the reservoir so as to transfer heat directly to the liquid with a high rate of heat transfer. An outlet from the reservoir is preferably so constructed as to reduce the amount of liquid carried over with the vapor and the outlet line may pass through at least one further closed vessel in order to trap any residual liquid. The internal combustion engine supplying the combustion products is preferably of the free-piston type and includes an arrangement for avoiding excessive heating of a release valve and its seat. For this purpose a duct is provided to supply liquid to a chamber in contact with the heated portion of the cylinder head and this chamber includes a portion adjacent the seat of the release valve which includes narrow outlets opening into the inlet duct in the region of reduced pressure immediately following the valve opening. In this way liquid is drawn into the inlet duct leading to appreciable cooling of the release valve and its seat.

United States Patent [191 May Sept. 25, 1973 VAPOR GENERATORS [76]Inventor: Claude H. May, Beaumont Cambridge Pk., St. Peter Port,

, Guernsey (Channel ls.)

[22] Filed: Nov. 19, 1971 [21] App]. No.: 200,445

[30] Foreign Application Priority Data Dec. 4, 1970 Great Britain57,809/70 [52] US. Cl 123/41.17, 123/25 D, 60/310, 126/360 A [51] Int.Cl. F0lp 9/02, F01n 3/04 [58] Field of Search 123/41.17, 188 S, 123/25D, 25 P; 122/31 A, 7 R; 126/360 A;

Primary Examiner-Al Lawrence Smith Assistant Examiner-Dennis TothAttorney-Joseph F. Brisebois et al.

[5 7] ABSTRACT Apparatus for the generation of liquid vapor underpressure, by means of heat from the combustion products of an internalcombustion engine comprises a reservoir for the liquid to be vaporizedhaving an inlet duct for the combustion products which communicates withthe lower part of the interior of the reservoir. The combustion productspass from the inlet duct into the reservoir by way of a screen formedwith a large number of small holes providing free passage for gas orliq-' uid and this screen is preferably formed by an extension of theduct lying within the reservoir and formed with the small holes roundits periphery. As a result of this construction the combustion productmay bubble upwardly through liquid in the reservoir so as to transferheat directly to the liquid with a high rate of heat transfer. An outletfrom the reservoir is preferably so constructed as to reduce the amountof liquid carried over with the vapor and the outlet line may passthrough at least one further closed vessel in order to trap any residualliquid. The internal combustion engine supplying the combustion productsis preferably of the freepiston type and includes an arrangement foravoiding excessive heating of a release valve and its seat. For thispurpose a duct is provided to supply liquid to a chamber in contact withthe heated portion of the cylinder head and this chamber includes aportion adjacent the seat of the release valve'which includes narrowoutlets opening into the inlet duct in the region of reduced pressureimmediately following the valve opening. In this way liquid is drawninto the inlet duct leading to appreciable cooling of the release valveand its seat.

5 Claims, 6 Drawing Figures United States Patent [1 1 1 3,760,778 MaySept. 25, 1973 r. 55 l 40 i5 l 42 A 39 I 4 0/ I as PATENTEDSEPZSIHTSSHEET 1 OF 4 Inventor Attorney PATENTEUSEPZSIHIS SHEET 2 BF 4 I nvenlorAttorney PATENTED8EP25|975 5.760.778

SHEET 3 (IF 4 Inventor Attorney PATENTED SEPZS I973 SHEET 0F 4 I rwenlorA Home y VAPOR GENERATORS This invention relates to apparatus for thegeneration of liquid vapour at high temperature and pressure forutilisation in driving prime movers. It is particularly applicable tothe generation of steam for such purposes although in'sorn'ecircumstances other liquids may be used. Aparticularly suitable sourceof heat for the generation of 'liquidvapour is'the combustion'productsof an internal combustion engine which are fed to a reservoir partlyfilled with liquid to be vaporised.

According to the present invention the reservoir has an inlet duct forthe combustion products which communicates with the lower part of theinterior ofthe'reservoir by way of a'screenfo'rmed with a large numberof smallholes providing free passage for gas or liquid whereby thecombustion product may bubble upwardly through liquid in the reservoirso'as to transfer heat'directly to the liquid. In this way, the heat andpressure of the combustion product may be transmittedtothe liquid withhigh efficiency. The screen is preferably constituted by an extension ofthe inlet duct lying within the reservoir and formed with thesmall'holes round its periphery. With such an arrangement it is possibleto create a large number of small bubbles so that the ratio of surfacearea to volume'isvery high and consequently a high rate of heat transferis achieved.

By selecting the size of hole so that the gas bubbles are above acertain critical size, the temperature of the trapped gas within thebubble will still be above that of the liquid when it breaks through thesurface. This will provide super heat to the vapour and in this way anext'remely efficient vapour generator is provided.

If the apparatus is to be fitted to a vehicle, for which purpose it isparticularly suitable, the reservoir is'prefe'rably cylindrical and isdivided into axial compartments by circular baffle plates formed withopenings for the passage of liquid and pressure fluid. The provision ofthese baffles avoids splashing and liquid surging during movement of theequipment while the presence of the openings permits free but controlledflow of both liquid and pressure fluid. The baffle plates may forexample divide the reservoir along its length into compartments having alength of about 65 percent of the diameter.

In use it is important to keep the vapour dry and it is thus desirableto arrange the outlet pipe from the reservoir so as to avoid as far aspossible liquid being carried over with the pressure fluid. For thispurpose the outlet may comprise a pipe which extends upwardly towardsthe top of the reservoir where its end is obstructed, the walls of thepipe close to the end being formed with openings for the passage ofpressure fluid and the upper portion of the pipe, including theopenings, being surrounded by a cup-shaped shield formed with drainageopenings at the bottom. Pressure fluid can pass freely over the top ofthe shield and out through the openings in the pipe, but any liquidwhich tends to be splashed over the top of the shield is unlikely toenter the pipe and merely flows outwardly through the drainage openings.As an additional protection, the drainage openings in the cup maythemselves be protected from splash by a down-turned skirt.

Apart from the construction of the reservoir itself, problems may arisein the internal combustion engine supplying the combustion productsowing to the very high temperature and back-pressure in the outlet ductof the engine and subsequent build-up of heat in the area of the releasevalve which leads to erosion of the valve and its seat. This excessiveheating may be at least partly counteracted by use of an engine which isspecifically designed for this purpose. In a preferred construction ofengine for this purpose an outlet from the, or each, release valve isconnected to the duct for the combustion products and a second duct isprovided to supplyliquid either from the reservoir or from a separatetank to a chamber or chambers in contact with the heated portion orportions of the, or each, cylinder head. Each chamber includes a portionadjacent the seatof the respective release valve and also includesnarrow outlets opening into the inlet duct in the region of reducedpressure immediately following the valve opening. In this way liquid isdrawn into the inlet duct, thus leading to a circulation of liquid fromthe reservoir, through the chamber, into contact with the metal adjacentthe seat of the release valve and then through.

the narrow openings into the inlet duct where the liquid is immediatelyvaporised by contact with the hot released gases. This leads toappreciable cooling of the, or each, release valve and the respectiveseat or seats.

Each chamber into which the liquid flows may include an' annular portionseparatedfrom the region of reduced pressure by an annular diaphragmhaving a free inner edge bearing against the wall of the annular portionof the chamber adjacent the valve seat, the narrow openings being formedbetween this edge of the diaphragm and the wall. The diaphragm thusdivides the chamber from the duct by which the combustion products aresupplied to the reservoir and additionally forms a sealing gasketagainst the high pressure of the pressure fluid.

Apparatus in accordance with the invention will now be described by wayof example with reference to the accompanying drawings, in which:

FIG. 1 is a vertical section through the end of a reservoir and of aninternal combustion engine supplying products of combustion to thereservoir;

FIG. 2 is a sectional view of the reservoir as seen from the right handside of FIG. 1;

FIG. 3 is an end view, partly in section, of the internal combustionengine seen from the left in FIG. 1;

FIG. 4 is a sectional view to a reduced scale corresponding to FIG. 2and showing subsequent details of an outlet line;

FIG. 5 is a detailed view showing part of a diaphragm seen in FIG. 1;and,

FIG. 6 is a sectional view to an enlarged scale showing part of arelease valve and seat seen in FIG. 1 and also including the edge of thediaphragm seen in FIG. 5.

. Turning first to FIG. 1, the release or outlet end of an internalcombustion engine is indicated generally as l and from it the productsof combustion pass along a line 2 and thence via an inlet duct 3 to theinterior of a cylindrical reservoir indicated generally as 4 and ofwhich only the left hand end is shown in FIG. 1. The inlet duct 3continues downwardly at 5 towards the bottom of the reservoir 4 where itturns through a right angle at 6 and leads to a perforated horizontalextension 7 formed with a large number of small holes 8 through whichthe products of combustion bubble upwardly through liquid in thereservoir 4. The level of the liquid is indicated by two horizontallines 9 which represent the upper and lower levels which may be assumedby the liquid during operation. For most purposes the liquid in thereservoir will be water and will be described as such, but under somecircumstances other liquids may be used.

The perforated extension 7 forms a screen through which passage of thecombustion products transfers heat directly to the water within thereservoir. The arrangement of holes 8 around the extension 7 is bestseen from FIG. 2, but this arrangement is by no means critical. Thearrangement is such that a large number of small bubbles is created sothat the ratio of surface area to volume is very high and consequently ahigh rate of heat transfer is achieved. When the liquid in the reservoiris water, it is found that a suitable size of hole is of approximately0.75 mm diameter which causes the gas bubbles to be above a criticalsize in which the temperature of the trapped gas is still above that ofthe water when each bubble breaks through thesurface. This providessuper heat to the steam and in this way an extremely efficient vapourgenerator is provided.

If the apparatus is fitted to a vehicle there is a risk of the watersurging from one end to the other of the reservoir and to reduce this,baffle plates 12 of which only one is seen in FIG. 2 are provided atintervals along the length of the reservoir. Each plate 12 is formedwith openings 13 seen in FIG. 2 which permits free but con trolled flowof both liquid and steam while avoiding splashing and the surgingmovement of the water. The baffle plates 12 are arranged to divide thereservoir 4 into compartments each having a length of about 65 percentof the diameter.

Outlet of pressure fluid from the reservoir 4 is controlled by therequirements of the motor to be driven or other apparatus to be suppliedwith the pressure fluid. An outlet pipe 15 extends from the top of thereservoir 4 and passes outwardly through the wall at 16 in the lowerhalf of the reservoir. It is important to avoid liquid being carriedover with the pressure fluid into the pipe 15 and for this purpose thepipe is formed close to its end (which is obstructed by the presence ofthe wall of the reservoir) with small openings 17, and the upper end ofthe pipe is surrounded by a cup-shaped shield 20 formed with drainageopenings 21 close to its bottom and lying on a line at right angles tothat of the openings 17. Although pressure fluid can flow freely throughthe entrance to the pipe 15, little water can get over the top of thecup 20 and such as does passes over in a direction at right angles tothe line of the openings 17 and then drains away through the openings 21rather than into the pipe 15. As an additional protection the drainageopenings 21 are themselves protected by a downturned skirt 22.

Despite these various precautions, there is still a risk that a smallquantity. of water may be carried over down the pipe 15 and this needsto be trapped and allowed to be vaporised by the higher temperature ofthe superheated steam which follows it. For this purpose, reservoirs 24and 34 (shown in FIG. 4) similar to the reservoir 4 are provided inseries with the outlet from the reservoir 4. In other words, pressurefluid from the reservoir 4 together with any entrained liquid passesfirstly to the reservoir 24, the outlet from which is arranged in asimilar way to that from the reservoir 4 and the components of which areindicated by the same reference numerals. The outlet from the reservoir24 then passes to the reservoir 34 where again the outlet arrangement issimilar, in any convenient direction and from there the pressure fluidpasses onwardly to its point of application. By the time the pressurefluid leaves the reservoir 34 any entrained liquid is quite negligible.

By means of the arrangements just described, the heat from and with theproducts of combustion of the engine 1 is converted with high efficiencyinto a pressure fluid which can be withdrawn as required. As thepressure in the reservoir 4 builds up, it applies back pressure to theengine 1 and this, in combination with the high temperatures involvedtends to cause the release valve of the engine to overheat, leading toerosion of the valve and its seat. Although any form of internalcombustion engine can be used for the supply of combustion products, afree-piston engine is particularly suitable for this purpose and it isan engine of this type which is shown in the drawings.

The piston and cylinder are shown as 35 and 36 respectively and therelease valve and its seat as 38 and 39 respectively. The release valvecarries a sparking plug 40 at its center and is controlled by fourloading springs 41 of the leaf pack type. Two of these are seen at theright hand side of FIG. 3, the left hand half of this Figure being insection. The valve 38 opens automatically when the pressure in thecombustion chamber shown as 42 exceeds that exerted by the springs 41and the combustion products then pass to the reservoir 4. Thefree-piston engine illustrated will, of course, have two opposedcylinders, but in principle an engine having any number of cylinders maybe used. These will all have similar release valve arrangements so thata description of the single cylinder shown in the drawing will suffice.

In the illustrated arrangement, the line 2, inlet duct 3,5 and rightangle piece 6, are repeated in mirror fashion at the opposite end of thereservoir 4, where the products of combustion enter the horizontalextension 7 from the opposite end. With any other number of cylindersthese may either be arranged to manifold into the existing lines 2, 3 ormay use an additional reservoir 4a shown in dotted lines in FIG. 4which, in turn, feeds into the reservoir 24.

In order to reduce erosion of the release valve 38 and its seat 39 acooling arrangement is provided which (as illustrated) is supplied withliquid from the reservoir 4 by a further line not shown in detail butindicated diagrammatically by the dotted line 45. Alternatively thisline may be supplied from a separate source to replace lost water.Before passing to a detailed description of the cooling system, it isimportant to understand the sequence of events immediately prior to andfollowing the instant of ignition in the combustion chamber 42 of thefree-piston engine. For purposes of description it can be assumed thatthe cylinder is freshly and fully charged with a stoichiometric air/fuelmixture and that the piston 35 is approaching the peak of compressionand ignition. At this stage, clue to the logarithmic characteristics ofcompression, the change in the rate of rise of pressure in thecombustion chamber 42 is quite rapid. As the kinetic energy previouslyimparted to the piston 35 is being expended equally rapidly,deceleration is correspondingly high. Just as the piston reaches thelimit of its outward stroke ignition occurs and almost immediatelythereafter several other events follow on virtually simultaneously. Theactual order of these events may not be that described because ofcertain operational variables, but in any case only a few microsecondswill separate the events and their interaction.

The rise of pressure in the combustion chamber 42 will absorb theremaining kinetic energy in the assembly including the piston 35 whichwill thus complete its outward stroke and instantaneously startaccelerating in the opposite direction, thus starting to increase thevolume of the combustion chamber 42. As it is virtually impossible forall the heat in the fuel to be yielded to the charge at constant volume,there is a time-lag between the generation of the flame nucleus at theignition point and flame propagation throughout the charge. Pressurethen rises rapidly and soon exceeds that exerted by the springs 41 sothat the release valve 38 starts to open. The products of combustionstart to escape at acoustic velocity which is modified by a coefficientof efflux due to the narrow orifice 46 between the valve 38 and the seat39. As the valve 38 opens to the full the coefficient will improve andthe orifice 46 will enlarge. Whereas in a conventional engine, ignitionin the closed chamber would cause a pressure rise of some five times thecompression pressure, this does not occur in a free-piston engine asillustrated in which the pressure rise is limited to about half thatwhich would otherwise be expected. Nevertheless, by the time the releasevalve 38 is again seated which is about 1 millisecond later, some 80percent of the combustion products will have escaped and most of theenergy necessary for cycling will have been imparted to the pistonassembly. At this stage the inward movement of the piston will have ledto an increase of volume of the combustion space 42 of about 60 percent.This space is of course filled with gas still at a fairly hightemperature but at a pressure only a little above the compressionpressure. As a result, the weight of the gas will be only about 10percent that of the initial charge. During the subsequent expansion andrapid cooling the remainder of the energy necessary for the cycling isimparted to the piston assembly and completion of the inward stroke thendrops the pressure in the cylinder below ambient atmospheric pressure.The result of the above further improves the overall thermal efficiencyof the unit quite apart from the high efficiency of the operation of thereservoir 4, as already described.

The improvement results from three main factors. Since the free-pistonengine operates substantially at one set load and frequency, it ispossible to adjust both the air/fuel ratio and the ignition timing totheir optimum settings. The total amount of heat for subsequent transferto the working fluid in the reservoir 4 is removed from the combustionspace 42 at the outer end of the working stroke of the piston and, asalready described, only the small proportion of burned gas required forcycling loses heat to the cylinder walls. Moreover, the relatively muchlower pressure of gas during this stroke results in less friction due toinflation of the piston rings. Finally, as a result of thecooling systemabout to be described, the unavoidable heat transfer through the wallsof the combustion chamber 42 is absorbed by the cooling liquid andpasses with it and the products of combustion into the pressure fluid inthe reservoir 4.

Returning to the details of the cooling system supplied by the line 45,the liquid from the reservoir 4 passes through a fitting (not shown inthe drawings) screwed into the outer head 51 at 52. From there theliquid passes to a substantially annular chamber 53.

From there the liquid flows circumferentially around the chamber 53 inboth directions as indicated by the arrow in the half-section of FIG. 3,most of this liquid passing through a pair of holes 54, one of which isseen in both FIGS. 1 and 3. The remainder passes through a hole 55,which reduces the risk of vapour lock. These holes all extend throughinto an annular chamber 60 surrounding the inner head 61. The chamber 60is separated by a diaphragm 62 from a chamber 63 into which the productsof combustion pass immediately after flowing through the orifice 46 whenleaving the combustion chamber 42 after the release valve 38 has opened.The holes 54 and 55 thus pass through an outer portion of the diaphragm62 which is annular in shape and has a free inner edge 65 which bearsagainst the inner head 61 which constitutes the wall of the chamber 60.The inner edge 65 of the diaphragm 62 is shown to an enlarged scale inFIG. 6 for which it can be seen that the edge locates in an annularrecess indicated generally as 67 in the inner head 61. This recess 67has a square-cut corner 68 and the edge 65 is chamfered at 70-so as toleave an annular passage 71 of triangular section between the edge ofthe diaphragm 62 and the recess 67. In addition, the edge 65 of thediaphragm 62 is shaped as shown in FIG. 5. Thus the right hand side ofthe diaphragm as seen in FIG 6, that is to say the side in communicationwith the chamber 60, is formed with a series of approximately radialslots 73, which decrease in depth from the edge 65 so as to form atriangular configuration as seen in FIG. 5. These slots are equallyspaced and alternate with a series of generally axial slots 74 best seenin FIG. 5.1m one of whichcan be seen in FIG. 6.

Thus it will be understood that the combination of the slots 73, theannular passage 71 and the axial slots 74 provides a series of narrowopenings connecting the liquid in the chamber 60 to the chamber 63. Inother words, liquid is able to pass in a generally radial directionalong one of the slots 73, from there in a circumferential directionalong the passage 71 and thence in an axial direction along one of theslots 74 into the chamber 63. The diaphragm 62 effectively forms asealing gasket between the liquid in the chamber 60 and the highpressure of the combustion products in the chamber 63 against leakage tothe exterior of the head. Nevertheless, the presence of the narrowopenings just described allows a small proportion of the liquid to passgradually through to the chamber 63 where it is swept up and vaporisedby the products of combustion as a result of the venturi or extractoreffect created by the high velocity flow through the orifice 46 and intothe widely expanding passage following it. The overall effect is thatliquid entering from line 45 firstly has a cooling effect in passingthrough the chamber 53, being itself further heated in the process, andthen, in passing into the chamber 60 absorbs conducted heat from thechamber walls and serves to scour excess heat from the vicinity of thevalve seat 39.

The large scale view of FIG. 6, in addition to showing the relationshipbetween the diaphragm 62 and the inner head 61, also shows furtherdetails of the release valve 38 and the seat 39. Both these parts aresubject to erosion due to the high temperatures involved since excessheat absorbed by the valve 38 will tend to flow to the seat 39 duringthe considerable portion of each cycle when the valve is seated. Inorder to reduce the efiects of erosion on the engaging faces of thesetwo components, each is shown as provided with an insert 80 and 81respectively of metal resistant to high temperature gas erosion andthermal shock. A particularly suitable metal for this purpose is acobalt base alloy such as that available under the Trade Mark Stellite6. The valve 38 slides in a guide bore indicated as 83 and the provisionof an adequate pressure seal between the valve 38 and the bore 83 is ofconsiderable importance. For this purpose, the wall of the bore isextended at 85 to form a relatively thin, sharp-angled lip which, duringthe initial manufacture is deformed inwardly as shown in dotted lines at86 so as to form an interference fit with the valve 38. During initialrunning, this interference fit maintains an adequate seal, but evenafter long periods of running when both the surface of the valve 38 andof the lip 85 have been subjected to wear, the seal is maintained sincethe pressure in the chamber 63 develops a hoop-stress in the lip 85which will keep the lip in close contact with the sides of the valve 38,thus providing an efficient seal with little friction and negligiblewear.

I claim:

1. Apparatus for the generation of liquid vapour under pressure by meansof heat from the combustion products of an internal combustion engine,said apparatus comprising an internal combustion engine having at leastone release valve having a seat and an outlet, a reservoir for theliquid to be vapourised, an inlet duct to said reservoir for saidcombustion products, and a connection between said outlet of saidrelease valve and said inlet duct for the combustion products, said ductcommunicating with the lower part of the interior of the reservoir, anda screen formed with a large number of small holes providing freepassage for gas or liquid, said screen being arranged between said ductand the interior of said reservoir whereby the combustion products maybubble upwardly through liquid in the reservoir so as to transfer heatdirectly to the liquid, and said engine including at least one cylinderhead and structure defining at least one chamber in contact with saidcylinder head, and a second duct connected to supply liquid from saidreservoir to said chamber, said chamber including a portion adjacentsaid seat of said release valve, said portion including narrow outletsopening into a region in said inlet duct immediately following saidvalve, whereby liquid is drawn from said chamber into said inlet ductdue to reduced pressure in said region.

2. Apparatus according to claim 1, in which said chamber includes anannular diaphragm mounted to separate said annular portion from saidregion in said inlet duct, said annular diaphragm having a free inneredge bearing against said cylinder head adjacent said valve seat, saidnarrow openings being formed between said inner edge of said diaphragmand said cylinder head.

3. Apparatus according to claim 2, in which said edge of said diaphragmlocates in an annular recess in said cylinder head, said recess beingformed with a relatively sharp corner when seen in section, said edge ofsaid diaphragm having a chamfered corner registering with said sharpcorner in said recess to leave an annular passage, the side of saiddiaphragm adjacent said annular portion of said chamber being formedwith a plurality of approximately radial slots connecting said portionof said chamber to said passage and said edge of said diaphragm beingformed with a plurality of approximately axial slots connecting saidannular passage to said region in said inlet duct, whereby thecombination of said radial slots, said annular passage and said axialslots together constitute said narrow openings.

4. Apparatus according to claim 1, in which engaging surfaces of saidrelease valve and said seat are provided with inserts of metal resistantto high-temperature gas erosion and thermal shock.

5. Apparatus according to claim 1, including a guide bore in which saidrelease valve works, said bore having a wall which is extended to form arelatively thin, narrow-angled lip bearing against the side of saidrelease valve to maintain an effective seal.

1. Apparatus for the generation of liquid vapour under pressure by meansof heat from the combustion products of an internal combustion engine,said apparatus comprising an internal combustion engine having at leastone release valve having a seat and an outlet, a reservoir for theliquid to be vapourised, an inlet duct to said reservoir for saidcombustion products, and a connection between said outlet of saidrelease valve and said inlet duct for the combustion products, said ductcommunicating with the lower part of the interior of the reservoir, anda screen formed with a large number of small holes providing freepassage for gas or liquid, said screen being arranged between said ductand the interior of said reservoir whereby the combustion products maybubble upwardly through liquid in the reservoir so as to transfer heatdirectly to the liquid, and said engine including at least one cylinderhead and structure defining at least one chamber in contact with saidcylinder head, and a second duct connected to supply liquid from saidreservoir to said chamber, said chamber including a portion adjacentsaid seat of said release valve, said portion including narrow outletsopening into a region in said inlet duct immediately following saidvalve, whereby liquid is drawn from said chamber into said inlet ductdue to reduced pressure in said region.
 2. Apparatus according to claim1, in which said chamber includes an annular diaphragm mounted toseparate said annular portion from said region in said inlet duct, saidannular diaphragm having a free inner edge bearing against said cylinderhead adjacent said valve seat, said narrow openings being formed betweensaid inner edge of said diaphragm and said cylinder head.
 3. Apparatusaccording to claim 2, in which said edge of said diaphragm locates in anannular recess in said cylinder head, said recess being formed with arelatively sharp corner when seen in section, said edge of saiddiaphragm having a chamfered corner registering with said sharp cornerin said recess to leave an annular passage, the side of said diaphragmadjacent said annular portion of said chamber being formed with aplurality of approximately radial slots connecting said portion of saidchamber to said passage and said edge of said diaphragm being formedwith a plurality of approximately axial slots connecting said annularpassage to said region in said inlet duct, whereby the combination ofsaid radial slots, said annular passage and said axial slots togetherconstitute said narrow openings.
 4. Apparatus according to claim 1, inwhich engaging surfaces of said release valve and said seat are providedwith inserts of metal resistant to high-temperature gas erosion andthermal shock.
 5. Apparatus according to claim 1, including a guide borein which said release valve works, said bore having a wall which isextended to form a relatively thin, narrow-angled lip bearing againstthe side of said release valve to maintain an effective seal.